Data from NASA's Curiosity rover enabled scientists to measure the total organic carbon content of rocks on Mars for the first time. According to Jennifer Stern of NASA's Goddard Space Flight Center in Greenbelt, Maryland, 'Total organic carbon is one of several measurements that tell us how much material is available for prebiotic chemistry and biology.'

She added, 'We found at least 200 to 273 parts per million of organic carbon. 'This is comparable to or even more than the amount found in rocks from very low-life places on Earth, such as parts of the Atacama Desert in South America, and it is more than that found in Martian meteorites.'

A carbon atom bound to a hydrogen atom is called organic carbon. All known forms of life rely on it for their creation and use of organic molecules. Since organic carbon can also come from nonliving sources, its presence on Mars does not prove that life exists there.

It can, for example, come from meteorites, or volcanoes, or be formed in place by surface reactions. Mars has previously been found to contain organic carbon, but previous measurements only provided information on specific compounds or measured only a portion of the carbon in the rocks. Our measurement measures the entire amount of organic carbon present in these rocks.

Mars is inhospitable for life today, but evidence suggests that billions of years ago, the climate was more Earth-like, with a thicker atmosphere and liquid water flowing into rivers and seas.

The lack of liquid water is a prerequisite for life on Earth, so scientists believe Martian life could have been sustained by key ingredients like organic carbon is present in a sufficient amount.

Through its investigation of Mars' habitability and study of its climate and geology, Curiosity is expanding the field of astrobiology. Samples were taken from mudstone rocks in the Yellowknife Bay formation of Gale Crater, the site of an ancient lake on Mars that is 3.5 billion years old.

At Gale Crater, mudstone was formed from very fine sediment (weathering of volcanic rocks) in water that settled on the bottom of a lake and was buried. This material contained organic carbon, which was incorporated into mudstone.

In addition to liquid water and organic carbon, Gale Crater featured chemical energy sources, low acidity, and other elements essential for life, such as oxygen, nitrogen, and sulfur. Stern, lead author of a paper about this research published June 27 in the Proceedings of the National Academy of Sciences, says if life ever existed at this location, it would have offered a habitable environment.

For the measurement, Curiosity delivered the sample to the Sample Analysis at Mars (SAM) instrument, which heated the powdered rock in an oven to increasing temperatures. In this experiment, oxygen and heat convert organic carbon to carbon dioxide (CO2), whose amount is measured to determine the amount of organic carbon in the rocks.

In the presence of oxygen and heat, carbon molecules break apart and react with oxygen to form CO2. The oven heats the sample to very high temperatures so that minerals are decomposed and carbon is released, allowing it to become CO2.

The experiment was conducted in 2014, but it took years to analyze the data and put the results in context with other discoveries the mission made at Gale Crater. Curiosity performed the resource-intensive experiment only once during its 10 years on Mars.

In addition to measuring the carbon isotope ratios, this process allowed SAM to determine the source of the carbon. There are various versions of an element with slightly different weights (masses) due to the presence of extra neutrons in the center of their atoms.

As an example, carbon-12 has six neutrons while carbon-13 has seven neutrons. The carbon from life is richer in carbon-12 because heavier isotopes tend to react more slowly than lighter isotopes. 'In this case, the isotopic composition can only tell us what part of the total carbon is organic carbon and what part is mineral carbon,' Stern said.

Even though biology cannot be completely ruled out, isotopes cannot really support a biological origin for this carbon, because the isotope range overlaps with those found in igneous (volcanic) carbon and meteoritic organic matter, which are most likely sources of this organic carbon.'

NASA's Mars Exploration Program funded the research. JPL, managed by Caltech, is NASA's Jet Propulsion Laboratory, which is leading Curiosity's Mars Science Laboratory mission. NASA's Goddard Space Flight Center in Greenbelt, Maryland, built and tested SAM. Malespin is the principal investigator for SAM.